The invention relates video and graphics display devices, to analog circuits for driving the picture elements (pixels) of video and graphics display devices, and, in particular, to analog circuits for driving the picture elements of a display device based on an electro-optical material.
A substantial need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. In particular, wearable display devices could be used instead of the conventional displays of laptop and other portable computers. Potentially, wearable display devices can provide greater brightness, better resolution, larger apparent size, greater privacy, substantially less power consumption and longer battery life than conventional active matrix or double-scan liquid crystal-based displays. Other potential applications of wearable display devices are in personal video monitors, in video games and in virtual reality systems.
Miniaturized displays based on cathode-ray tubes or conventional liquid crystal displays have not been successful in meeting the demands of wearable displays for low weight and small size. Of greater promise is a micro display of the type described in U.S. Pat. No. 5,596,451 of Handschy et al., the disclosure of which is incorporated into this disclosure by reference. This type of micro display includes a reflective spatial light modulator that uses a ferroelectric liquid crystal (FLC) material as its light control element.
The spatial light modulator of the FLC-based micro display just described is driven by a digital drive signal. The conventional analog video signal generated by the graphics card of a personal computer, for example, is fed to a converter that converts the analog video signal into a digital bitstream suitable for driving the spatial light modulator. The converter converts the analog video signal into a time domain binary weighted digital drive signal suitable for driving the spatial light modulator. The time durations of the bits of the time domain binary weighted digital drive are binary weighted, so that the duration of the most-significant bits is 2n-1 times that of the least-significant bits, where n is the number of bits representing each sample of the analog video signal. For example, if each sample of the analog video signal is represented by 8 bits, the duration of each most-significant bit is 256 times that of each least-significant bit. Driving the pixels digitally means that the pixel driver must be capable of changing state several times during each frame of the analog video signal. The switching speed must be shorter than the duration of the least-significant bit. This requires that the drive circuitry in each pixel be capable of high-speed operation, which increases the power demand and expense of the micro display system. On the other hand, the long time duration of the most-significant bits of the digital drive signal means that the digital drive signal is static for the majority of the frame period.
Practical embodiments of the micro display referred to above typically locate the converter referred to above external of the micro display and connect the converter to the micro display by a high-speed digital link. The converter time multiplexes the digital drive signals for transmission though the digital link as follows: the least-significant bits for of the digital drive signals all the pixels of the spatial light modulator, followed by the next-least-significant bits of the digital drive signals for all the pixels, and so on through the most-significant bits of the digital drive signals for all the pixels. The digital link must be capable of transmitting all the bits representing each frame of the component video signal within the frame period of the component video signal. The digital link, its driver and receiver must be capable of switching at a switching speed shorter than the duration of the least-significant bit, yet remain static for times corresponding to the durations of the most-significant bits.
In addition, the converter requires a large, high-speed buffer memory to convert the parallel, raster-scan order digital signals generated from the analog video signal to a bit-order signal for each color component. This increases the cost and power requirements of the converter.
The digital serial link can be eliminated by locating the converter in the micro display itself, but relocating the converter increases the size, weight and complexity of the micro display. Moreover, miniaturizing the converter to fit it in the micro display can increase the cost of the converter. Finally, relocating the converter does not reduce its overall cost and complexity.
What is needed is a miniature display device that can operate in response to a video signal or graphics data and that does not suffer from the size, weight, complexity and cost disadvantages of the conventional digitally-driven micro display.
Conventional-sized video and graphics displays rely on cathode-ray tubes or full-size liquid crystal displays. The former are bulky, heavy and fragile. The former are also expensive to produce and are very heavy in the larger sizes required to realize the benefits of high-definition video. The latter are expensive to produce in screen sizes comparable with conventional cathode-ray tubes, and have a limited dynamic range and a limited viewing angle. What is also needed is a miniature display device that can form the basis of an full-size video and graphics display that would provide an effective alternative to conventional cathode-ray tubes and liquid crystal displays.
The invention provides a display device based on an electro-optical material. The display device operates in response to an information signal and comprises analog drive circuits arranged in a two-dimensional array of rows and columns, an analog sampling circuit that derives the analog samples from the information signal, and a sample distribution circuit. The sample distribution circuit receives the analog samples from the analog sampling circuit and distributes the analog samples to the analog drive circuits. The sample distribution circuit includes input gates corresponding to the analog drive circuits, column busses corresponding to the columns of the array, and a row selector having outputs corresponding to the rows of the array. The column busses perform a column-wise distribution of the analog samples to the analog drive circuits. The analog drive circuits are connected to the column busses by the input gates. Each of the outputs of the row selector is connected to control the input gates in one of the rows. The row selector sequentially opens the input gates in the rows to perform a row-wise selection of the analog samples on the column busses.
The analog sampling circuit may include a sampling circuit and a column selector. The sampling circuit comprises a row of sample-and-hold circuits. Each of the sample-and-hold circuits corresponds to one of the column busses and comprises an output connected to the one of the column busses, an input connected to receive the information signal, and a column control signal input. The column selector is connected to the column control signal inputs of the sample-and-hold circuits. The column selector generates column control signals for the sample-and-hold circuits at a signal rate related to the information signal. The column control signal for a one of the sample-and-hold circuits is in an opposite state to the column control signals for the remaining ones of the sample-and-hold circuits. The column control signal in the opposite state moves progressively along the row of sample-and-hold circuits at the signal rate.
When the information signal is a color video signal, the analog sampling circuit and sample distribution circuit may both include serial or parallel arrangements to derive and distribute analog samples of the color components of the color video signal to the analog drive circuits.
When the information signal is a video signal composed of lines and frames, the location in each of the lines of the video signal from which the analog sampling circuit derives the analog samples that the sample distribution circuit distributes to each column bus depends on the location of the column bus in the array.
The invention also provides a display device based on an electro-optical material. The display device operates in response to an information signal, and comprises an array of pixels, a sample distribution circuit and a light source. Each of the pixels includes an electrode electrically coupled to the electro-optical material, and an analog drive circuit that includes an output electrically connected to the electrode. The sample distribution circuit distributes an analog sample derived from the information signal to the analog drive circuit of each of the pixels. The analog drive circuit generates a drive signal composed of a sequence of a first temporal portion and a second temporal portion, the first electrical portion having a time duration that has a predetermined relationship to the analog sample, the second temporal portion being a temporal complement of the first temporal portion. The light source illuminates the electro-optical material simultaneously with the analog drive circuit generating the drive signal sequence.
The analog drive circuit may include a sample selection section that stores the analog sample received from the sample distribution circuit, and a drive signal generator that generates the drive signal in response to the analog sample stored in the sample selection section.
The sequence of the first temporal portion and the second temporal portion may be a first sequence of the first temporal portion and the second temporal portion in which the analog drive circuit generates the drive signal in a first electrical state during the first temporal portion and in a second electrical state during the second temporal portion. The analog drive circuit may generate the drive signal additionally composed of a second sequence of the first temporal portion, in which the drive signal is in the second electrical state, and the second temporal portion, in which the drive signal is in the first electrical state. The first temporal portion and the second temporal portion may be in any order in the second sequence. In this case, the light source illuminates the electro-optical material during the first sequence.
The invention also provides a method of generating a grey scale in response to an information signal. The grey scale is generated by modulating light using an electro-optical material. In the method, an analog sample is derived from the information signal, a drive signal is generated in response to the analog sample, and the drive signal is applied to the electro-optical material. The drive signal generated in response to the analog sample includes a sequence of a first temporal portion and a second temporal portion. The first temporal portion has a time duration that has a pre-determined relationship to the analog sample, and the second temporal portion is the temporal complement of the first temporal portion.
The method may additionally comprise illuminating the electro-optical material in synchronism with the drive signal.
Generating the drive signal may be subject to an error factor that changes the predetermined relationship between the duration of the first temporal state and the analog sample, and the method may additionally comprise minimizing the visual effect of the error factor on the gray scale. When the information signal includes odd-numbered sequences interleaved with even-numbered sequences, such as in a video signal, minimizing the visual effect of the error factor on the gray scale may include inverting the sense of the error factor when generating the drive signal in response to either the odd-numbered sequences or even-numbered sequences.